A surface that is maintained at a temperature colder than its environment removes (i.e., "pumps") gases from the environment by the physical processes of condensation and/or adsorption. This pumping on a cold (i.e., "cryogenic") surface is termed "cryopumping" from the Greek word "kryos" meaning "cold". The process of condensation on a cryogenic surface is termed "cryocondensation", and the process of adsorption on a cold surface is termed "cryosorption".
The present invention pertains to a closed-cycle, two-stage cryogenic pumping apparatus. In operation, the first pumping stage is typically maintained at a cold temperature in the range, e.g., from 50.degree. K. to 80.degree. K., and the second pumping stage is maintained at a colder temperature in the range, e.g., from 10.degree. K. to 20.degree. K. The first pumping stage is generally configured to provide gas conductance passages whereby gas species that impinge upon but are not cryopumped at the first stage can pass through the first stage to the colder second stage. Thus, gases such as water vapor and carbon dioxide, whose heats of condensation, heats of solidification and specific heats are such that their cryocondensation can occur at the higher first-stage temperature, are pumped at the first stage; and gases that require a lower temperature for cryocondensation or cryosorption (e.g., helium, hydrogen and neon) are pumped at the second stage.
It is a primary function of the first pumping stage to remove substantially all the gases that can be cryopumped at the first-stage operating temperature, so as to prevent such gases from loading the surfaces of the colder second pumping stage. In this way, the capacity of the second stage for cryopumping the remaining gases can be maximized. Another function of the first stage is to shield the colder second stage from thermal radiation that would reduce the usable refrigeration capacity of the second-stage surfaces for pumping gases.
In a two-stage cryogenic pumping system, the second pumping stage is often configured as a structure having a smooth exterior surface and an interior surface that is coated with a cryosorbent material such as activated charcoal or an artificial zeolite. With such a second-stage structure, gases such as helium, hydrogen and neon that do not readily cryocondense on the relatively smooth exterior surface can be cryosorbed in the interstices of the coating material on the interior surface.
A detailed overview of cryogenic pumping is provided in an article entitled "Helium Cryopumping" by Kimo M. Welch and Chris Flegal, which was published in Industrial Research/Development, March 1978, pages 83-88. The illustration appearing on page 83 of that article shows a typical prior art cryogenic pump configuration. In that typical configuration, the first pumping stage comprised a cup-like structure having solid side and bottom walls, with a panel of chevroned baffles forming a planar cover at the upper end; and the second pumping stage was enclosed within the first pumping stage and had the form of an inverted cylindrical cup with an open bottom end.
A problem associated with cryogenic pumps of the prior art was their low pumping speed. Whatever configuration such prior art pumps may have had for the first-stage pumping structure gas transmission to the second stage was generally possible only through a very limited surface area on the first-stage structure. For the typical prior art configuration illustrated in the above-cited "Helium Cryopumping" article, for example, the panel of chevroned baffles of the first-stage pumping structure permitted gas transmission to the second stage only in two dimensions through the plane at the top of the cup-like first-stage structure. Furthermore, with that typical prior art configuration, gas species passing through the first-stage baffles could reach the interior surfaces of the second-stage pumping structure only via a tortuous path downward through the annular region between opposing surfaces of the two pumping stages and thence into the interior of the second-stage structure.